Fair Value Measurements - Fair Value by Balance Sheet Grouping (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Sep. 30, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	$ 1,693,047	$ 1,485,742
Held-to-maturity securities	948,193	1,463,095
FHLB and FRB stock	155,990	123,990
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	1,693,047	1,485,742
Held-to-maturity securities	920,255	1,443,480
Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	1,693,047	1,485,742
Held-to-maturity securities	948,193	1,463,095
Level 1 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1,495,574	419,158
Level 1 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1,495,574	419,158
Level 2 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
FHLB and FRB stock	155,990	123,990
Time deposits	4,424,382	4,906,963
FHLB advances	3,050,000	2,250,000
Level 2 \| Carrying Amount \| Client swap program hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	43,157	20,381
Other liabilities	43,157	20,381
Level 2 \| Carrying Amount \| Mortgage loan fair value hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	0	1,608
Other liabilities	13,961	0
Level 2 \| Carrying Amount \| Commercial loan fair value hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	8,703	4,288
Level 2 \| Carrying Amount \| Borrowings cash flow hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	17,189	7,877
Level 2 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
FHLB and FRB stock	155,990	123,990
Time deposits	4,451,266	4,937,847
FHLB advances	3,023,003	2,282,887
Level 2 \| Estimated Fair Value \| Client swap program hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	43,157	20,381
Other liabilities	43,157	20,381
Level 2 \| Estimated Fair Value \| Mortgage loan fair value hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	0	1,608
Other liabilities	13,961	0
Level 2 \| Estimated Fair Value \| Commercial loan fair value hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	8,703	4,288
Level 2 \| Estimated Fair Value \| Borrowings cash flow hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	17,189	7,877
Level 3 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans receivable	11,974,241	11,930,575
Level 3 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans receivable	12,328,962	12,617,600
U.S. government and agency securities \| Level 2 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	338,361	270,778
U.S. government and agency securities \| Level 2 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	338,361	270,778
Municipal bonds \| Level 2 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	21,997	22,642
Municipal bonds \| Level 2 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	21,997	22,642
Corporate debt securities \| Level 2 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	259,524	209,763
Corporate debt securities \| Level 2 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	259,524	209,763
Agency pass-through certificates
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	941,181	1,448,088
Agency pass-through certificates \| Level 2 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	1,073,165	982,559
Held-to-maturity securities	913,243	1,428,480
Agency pass-through certificates \| Level 2 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	1,073,165	982,559
Held-to-maturity securities	941,181	1,448,088
Commercial MBS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	7,012	15,007
Commercial MBS \| Level 2 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	7,012	15,000
Commercial MBS \| Level 2 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	$ 7,012	$ 15,007

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Fair value portion of deposit liabilities held by the entity, including, but not limited to, foreign and domestic, interest and noninterest bearing, demand deposits, saving deposits, negotiable orders of withdrawal (NOW) and time deposits.

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Federal Home Loan Bank (FHLB) stock represents an equity interest in a FHLB. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

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Fair value portion of other liabilities.

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Federal Home Loan Bank Advances, Long Term and Other Borrowings, Fair Value Disclosure

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